Photoresponsive materials

ABSTRACT

A photoresponsive material comprises molecular or polymeric semiconductors in which photogeneration of separated charge carriers proceeds substantially via an intermediate stage of a triplet exciton, where the generation of the triplet exciton may be facilitated by the presence of elements of high atomic number, and in which the generation of separated charges from the intermediate triplet excited states is facilitated by the presence of at least two semiconductive components, one of which is of high electron affinity and able to accept electrons, and the other of which is of low ionisation potential and therefore able to accept positive charge carriers, the difference between the electron affinity of the former and the ionisation potential of the latter being sufficiently low so as to allow the ionisation of a triplet exciton which is present on either of the two aforementioned semiconductive components or on a third component. The provision of contact electrodes allows charge to be collected by an external electrical circuit and allows light to be incident on the active photoresponsive material.

FIELD OF THE INVENTION

This invention relates to photoresponsive materials and to their use inphotoconductive and photovoltaic devices.

BACKGROUND OF THE INVENTION

The photogeneration of separated electronic charges in semiconductivematerials has long been studied as a means for the interconversion oflight (particularly solar) energy into electrical energy (thephotovoltaic effect) and, with the application of an external electricalbias, the generation of an electrical current to allow detection ofincident light (the photoconductive response). One of the manydifficulties in obtaining efficient generation of charge followingphotoexcitation is that the separated negative charges (electrons) andpositive charges (holes) may recombine to form a neutral excited statebefore they can be collected by the external electrodes. Anotherdifficulty is that in order to absorb all the light incident on thesemiconductive material, its thickness may be too great to allow easytransport of the electrons and holes to their respective electrodes, andthe thickness may be so great as to allow considerably opportunity forcapture of electrons and holes travelling past one another in the bulkof the material.

These difficulties are particularly serious in semiconductive materialswhich comprise molecular or polymeric semiconductors in which the mobilecharge carriers are contained in orbitals which involve carbon piorbitals. This is in part because the binding energies of the electronand hole can be considerably higher than the thermal energy at roomtemperature, such that dissociation of charge carriers is difficult andrecombination easy. A further problem is that the mobility of one orother charge carrier can be very low, such that movement of such chargecarriers across the full thickness of the semiconductive layer isdifficult to achieve.

One such semiconductive conjugated polymer is poly(para-1,4 phenylenevinylene), PPV. This polymer can be formed as thin films by a number ofmethods including solution processing of soluble precursors, or, withsuitable attachment of side-groups such as alkoxy groups attached at the2 and 5 positions at the phenylenes, can be directly processed fromsolution. The photoconductive and photovoltaic properties of thispolymer have been reported at the International Symposium onElectroluminescence in Polymers, Eindhoven, The Netherlands, 15th Sep.1993, and in "The Photovoltaic Response in Poly(p-phenylene vinylene)Thin Film Devices", R. N. Marks, J. J. M. Halls, D. D. C. Bradley, R. H.Friend and A. B. Holmes; J. Phys. Condensed Matter 6, 1379-1394 (1994).It is considered there that photogeneration of charge carriers resultsfrom ionisation in the bulk of the polymer of the singlet excitonsformed by photoexcitation, but this process is not intrinsicallyefficient when measurements are made in the photovoltaic mode (biasvoltages between the open circuit voltage of the device and zero volts),with values of no more than a percent for the quantum efficiency ofphoton to charge conversion for devices using optimised electrodes(calcium for electron collection in this mode), and considerably lowerwhen using more stable electrodes in this role which are less wellmatched in terms of their electronic properties, such as aluminium.

Properties have also been reported for the dialkoxy derivatives of PPV,for instance in "Dual-Function Semiconducting Polymer Devices:Light-Emitting and Photodetecting Diodes", G. Yu, C. Zhang and A. J.Heeger, Appl. Phys. Lett. 64, 1540-1542 (1994), and "SemiconductingPolymer Diodes: Large size, Low Cost Photodectors with ExcellentVisible-Ultraviolet Sensitivity", G. Yu, K. Pakbaz and A. J. Heeger,Appl. Phys. Lett 64, 20 Jun. 1994 (1994).

SUMMARY OF THE INVENTION

The aim of the present invention is to increase the efficiency of chargegeneration following photon absorption. According to an aspect of thepresent invention this is achieved by increasing the lifetime of anintermediate neutral excited state, by causing it to cross from thesinglet spin manifold in which it might have been formed to the spintriplet manifold, and then causing the triplet exciton to be ionised toenhance charge separation. Since radiative transitions of the tripletexciton to the ground electronic state, which is in the spin singletmanifold, are very slow, the triplet excited state can survive for verymuch longer than the singlet excited state before radiative decay. Thiscan allow it more time to undergo ionisation, and thus substantiallyraise the efficiency of the charge separation process. In one embodimentof the present invention a photoresponsive material is provided whichcontains a polymer or molecule which contains an element of high atomicnumber which is closely coupled to the molecular wavefunctions which areused to provide the semiconductive properties. The presence of such ahigh atomic number element can cause mixing of the singlet and tripletmanifolds, by action of the spin-orbit coupling interaction, and thiscan then allow efficient crossover from the singlet excited statesgenerated by photoexcitation to the intermediate triplet excited state.

One possible difficulty in the use of triplet excitons is that they arecommonly found to show higher binding energies against excitation toform separated electrons and holes than singlet excitons, and thus mayprove difficult to use as intermediates in the photogeneration of chargecarriers. To overcome this, the photoresponsive material includes twosubstances, one of high electron affinity, and the other of lowionisation potential to provide respectively electron and hole acceptingfunctions. It will be clear that the polymer or molecule which containsthe element of high atomic number may also provide one or other of thesetwo functions. Accordingly, a further embodiment of the inventionprovides a photoresponsive device containing at least two semiconductivecomponents which may be in the form of separated but adjacent layers, orelse in the form of a blended composite material, with the twosemiconductive components selected so that one is of high electronaffinity and therefore able to accept electrons, and the other is of lowionisation potential and therefore able to accept positive chargecarriers, the difference between the electron affinity of the former andthe ionisation potential of the latter being sufficiently low as toallow the ionisation of a triplet exciton which is present on either ofthe two aforementioned components or on a third component.

A further aspect of the invention is the manufacture of photoresponsivedevice having the aforementioned features in semiconductive materialswhich can be conveniently formed as thin films over large areas by theuse of solution-processing of soluble materials which are film-forming.Among these, polymers are selected which contain substantial fractionsof conjugated units (i.e. units in which there are pi molecular orbitalsformed on carbon atoms which are delocalised over the full extent of themolecular unit), such polymers being capable of providing both thefilm-forming properties of the composite and also at least some of thesemiconductive, charge-transporting properties.

The photogeneration of separated charges in the bulk of such materialsis generally considered to be an inherently difficult process on accountof the high binding energy of the electon and hole. Easier chargeseparation can be achieved if two semiconductive materials are in closeproximity and arranged so that one is better able to receive an electronand the other the hole, with the difference in electron affinity for theformer and ionisation potential for the latter arranged to be less thanthe energy of the bound exciton. Elegant demonstrations of the efficientcollection of charges produced from photons absorbed close to theinterface between two such semiconductive layers have been made usingmolecular semiconductors, as reported in "Two-Layer Organic PhotovoltaicCell", C. W. Tang, Appl. Phys. Lett 48, 183-185 (1986). This device islimited in efficiency by the requirement to make the semiconductivelayers sufficiently thick to absorb the incident photons, but as thesame time thin enough to allow excitons generated throughout thethickness of these layers to diffuse to the interface at whichionisation can proceed. With the short lifetimes found for singletexcitons, of no more than a few nanoseconds in materials of this, thediffusion range is typically no more than a few tens of nanometers, andtypically a factor of 10 less than the thickness required for fullabsorption.

These problems can be overcome by providing a material in whichcrossover to the triplet manifold is encouraged and in which theresulting triplet exciton is then ionised to render charge separationmore efficient.

In work done recently, for example as described in "Optical Spectroscopyof Platinum and Palladium Containing Poly-ynes", H. F. Wittmann, R. H.Friend, M. S. Khan and J. Lewis, J. Chem Phys. 101, 2693 (1994), it isshown that polymers containing transition metals such as platinum andpalladium within the main polymer chain can show semiconductiveproperties and can show very efficient intersystem conversion fromsinglet excited states to triplet excited states. These polymers areformed by the complexation of the metals in a square-planarconfiguration, with acetylenic groups forming ligands to oppositevertices of the square, and tributyl phosphonium groups forming theother two ligands. Connection of acetylenic groups to different metalsites via conjugated spacers such as para-phenylene provides the polymerstructure. Detection of the formation of triplet excitons is made invarious ways, including the measurement of phosphorescence (i.e. tripletto singlet ground state luminescence) which can be extremely efficientin these materials. This phosphorescence is efficient because the matrixelement connecting excited and ground states is considerably enhanced onaccount of the presence of the heavy metal atoms which provide strongspin-orbit coupling so that the radiative lifetimes for polymers formedwith the metal chosen as platinum and palladium are 30 and 500microseconds respectively.

Evidence that these polymers can show semiconductive properties ispresented in "The photovoltaic effect in a Platinum Poly-yne", A.Kohier, H. F. Wittmann, R. H. Friend, M. S. Khan and J. Lewis, presentedat the meeting of the European Materials Research Society, Strasbourg,France, 23-26 May 1994, and to be published in Synthetic Metals.However, only a very weak photoconductive response can be obtained whenthis polymer is sandwiched between electrodes of indium/tin oxide andaluminium, with quantum efficiency typically no more than 0.01%.

These metal-containing poly-ynes form one of many such molecular orpolymeric materials in which heavier elements are coupled into theexcited states of semiconductive materials. Besides the very extensiverange of transition metals which can be coupled to form square-planar oroctahedrally-coordinated complexes of this type, other materials whichincorporate bromine or iodine groups, metal-containing porphyrins andphthalocyanines, and metallocenes coupled to form delocalised pielectron systems are also suitable as materials which permit efficientcrossover from the singlet to the triplet excited state.

The present inventors discovered that the photoresponsive efficiency ofmaterials of this type can be remarkably increased by the inclusion of asubstance having a high electron affinity, which causes the tripletexcitons to be ionised and thus increase the efficiency of the chargeseparation process. A particularly suitable material having a highelectron affinity is fullerene, C₆₀. Other materials, selected to havehigh electron affinities, and which might therefore also be used in thisrole are conjugated polymers with electron withdrawing groups either inthe main chain or attached as side-groups. Examples of the former arepoly(pyridine) and poly(pyrazine), and poly(pyridine vinylene) andpoly(pyrazine vinylene). Examples of the latter include derivatives ofpoly(p-phenylene vinylene) with nitrile groups attached to the mainchain, at, for example, the vinylic carbons, as described in "EfficientPolymer-Based Light-Emitting Diodes. Based on Polymers with HighElectron Affinities", N. C. Greenham, S. C. Moratti, D. D. C. Bradley,R. H. Friend and A. B. Holmes, Nature 365, 628-630 (1993). Otherexamples are molecules or polymers containing electron withdrawinggroups such as oxazole and oxadiazole.

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a digram of a photoresponsive device.

FIG. 2 shows the molecular structure of a platinum poly-yne, refered toas the Pt-poly-yne.

FIG. 3 shows the spectral response of the photocurrent of platinumpolymer measured under short-circuit conditions in a device with ITO andaluminium electrodes. The light was incident from the ITO side. Theactual quantum efficiency (=number of archieved charge carriers/numberof incident photons) is plotted on the vertical axis.

FIG. 4 shows the same spectrum for a Pt-Poly-yne/C₆₀ Blend (Pt:C₆₀=10:1), also sandwiched between indium/tin oxide and aluminiumelectrodes.

FIG. 5 compares the photoluminescence spectra of the device with onlyPt-poly-yne and the device with a Pt-Poly-yne/C₆₀ blend (PtC:₆₀ =15:1).The sample containing C₆₀ was annealled at 160° C. for 10 hours.

FIG. 6 displays the same information for a device as described in FIG. 4but before the annealing of the blend device (PtC:₆₀ =15:1).

FIG. 7 shows the effect of annealing the Pt-poly-yne/C₆₀ blend(PtC:₆₀=15:1) device on the photoluminescence. The data for the annealleddevice are scaled up by a factor of 16.

FIG. 8 shows the effect of increasing the C₆₀ proportion of the blend.The samples are not annealled but they were kept under inert nitrogen orvacuum after spin-coating. Proportions of Pt-Poly-yne and C₆₀ are asindicated in the figure.

FIG. 9 shows the optical absorption spectra for films of the Pt-poly-yneand also the blend (PtC:₆₀ =10:1).

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a photoresponsive device according to one embidiment of theinvention. To fabricate a photoresponsive device of the type shown inFIG. 1, polymer layer 2 is spin-coated onto an electrode 4 of metal,here indium/tin oxide which is already deposited on a glass substrate 6.After deposition of the polymer layer 2, a further top electrode 8 ofaluminium is vacuum evaporated to form the working device. The polymerlayer 2 includes a photoresponsive material which contains an element ofhigh atomic number which is closely coupled to the molecularwavefunctions which are used to provide the semiconductive properties.The presence of such a high atomic number element can cause mixing ofthe singlet and triplet manifolds, by action of the spin-orbit couplinginteraction, and this can then allow efficient crossover from thesinglet excited states generated by photoexcitation to the intermediatetriplet excited state.

FIG. 2 shows the structure of a polymer, Pt-poly-yne, forming thepolymer layer and containing the heavy element, here platinum, and whichalso functions as a hole-collecting semiconductive component for theseexamples. This polymer is selected from the transition metal poly-ynesso that it can be processed in solution in toluene, and is spin-coatedfrom solution to form films of thicknesses of the order 100 nm. Thinfilms of the polymer were also spin-coated directly onto glasssubstrates to provide samples suitable for measurements ofphotoluminescence.

To cause ionisation of the triplet excitons, a further substance of highelectron affinity is provided. In the present example this substance isfullerene, C₆₀. This Pt-poly-yne is cosoluble with the fullerene C₆₀,and mixtures of the two were obtained by spin-coating the codissolvedmaterials from solution in toluene. The fullerene used for this purposewas a mixture of C₆₀ /C₇₀ of nominal composition 98% C₆₀. Similarmixtures with other components could also be obtained in this way, andalternative electron-accepting materials could be chosen in place ofC₆₀. These would include nitrile derivatives of PPV selected to showhigh electron affinities.

In the operation of the device described above, light is incidentthrough the indium/tin oxide electrode 4 onto the semiconductive polymerlayer 2. Spectrally-resolved light is produced by selecting a pass bandof wavelengths from a tungsten lamp 10 using a grating monochromator(Bentham 1/4 Meter). Electrical measurements were made using standardlaboratory voltage, sources and picoammeters (such as manufactured byKeithley Instruments). For the measurements we mention below, the devicewas operated under short-circuit conditions with the two electrodes heldat the same potential and with measurement of the current flowingbetween these two electrodes. In FIG. 1, block 12 generally denotes theelectrical equipment.

FIG. 3 shows the photovoltaic quantum efficiency under short circuitconditions for a device made with Pt-poly-yne. We note that a responseis seen above the threshold for photoexcitation of the singlet excitons,for photons with energies above 3 eV. Note that the peak efficienciesare no more than 1.2×10⁻⁴ electrons per absorbed photon. FIG. 4 showssimilar spectra, but now measured for a device formed with a 10:1 blendof Pt-poly-yne to C₆₀. Note that the spectral form of the response issimilar in shape, indicating that the principal response is dueinitially to excitation of singlet states in the Pt-poly-yne. Note alsothat the peak efficiency is now more than a factor of 100 greater thanfor the device formed with the polymer alone, reaching a value of 1.6%,and exceeding the efficiencies measured for devices of this type madewith single layers of other polymers such as PPV when sandwiched betweenthis combination of electrode metals.

FIG. 5 shows the photoluminescence spectra measured with photoexcitationwith the UV lines of an argon ion laser (near 360 nm), of both thePt-poly-yne by itself, and of a blend formed with C₆₀ in the ratioPt-poly-yne:C₆₀ =15:1. This blend was annealled for 10 hours at 160° C.in vauuo. For the pure polymer, strong luminescence bands are seenpeaking at 3.2 eV and at 2.5 eV, which are due to radiative decay ofsinglet and triplet excitons respectively. In the blend formed with C₆₀both luminescence bands are strongly quenched, with the triplet bandbeing more completely quenched. We attribute this quenching to thecharge transfer process from Pt-poly-yne to C₆₀. FIG. 6 shows similardata, comparing again the pure polymer with a similar blend which hadnot been annealled. For this sample, the singlet emission shows littlequenching, but the triplet emission is strongly quenched. It seems thatthe annealing process increases the ability of the C₆₀ to acceptelectrons from the Pt-poly-yne. FIG. 7 compares the luminescence datafor the sample before and after annealing, with the data for theannealled sample scaled by a factor of 16. This shows again very clearlythat the triplet emission is quenched much more effectively than thesinglet emission. This is consistent with the larger diffusion rangeexpected for the longer lived triplet excitons.

FIG. 8 shows photoluminescence data on blends formed with a range ofcompositions. The samples were not annealled, but were kept undernitrogen or vacuum after spin-coating. The spectra for compositionratios of Pt-poly-yne:C₆₀ of 1:1 and 15:1 are very similar, though thereis some evidence that the triplet emission is weaker in the 1:1composition.

FIG. 9 displays the optical absorbance defined as--logarithm(opticaltransmission) versus photo energy for the Pt-poly-yne and for a blend ofPt-poly-yne:C₆₀ =10:1. This shows evidence for weak absorption due tothe C₆₀ in the energy range below the onset of strong absorption in thePt-poly-yne at 3.0 eV.

The results presented in the series of FIGS. 3-9 provide clear evidencethat the mixtures of the Pt-poly-yne with C₆₀ show quenching of thetriplet luminescence and enhancement of carrier photogeneration, by afactor in excess of 100, over devices made with the Pt-poly-yne alone.

It will be appreciated that the device illustrated in FIG. 1 can be usedas a photoconductive or photovoltaic device by suitably selecting theelectrical circuitry in block 12 in a manner known to a person skilledin the art. Suitable applications include photodetectors including arraydetectors and photovoltaic cells.

It will also be readily appreciated that the method described formanufacturing the photoresponsive device makes us of the film-formingproperties of these polymers, by spin-coating from solution, forexample.

There has been described a photoresponsive material in whichphotogeneration of separated charge carriers proceeds substantially viaan intermediate stage of a triplet exciton, the photoresponsive materialcontaining molecular or polymeric semiconductors. The interconversion ofsinglet and triplet excited states is facilitated by the presence of anelement of high atomic number. Ionisation of the triplet excited stateis facilitated by a component of high electron affinity and a componentof low ionisation potential, the difference between the ionisationpotential of the latter and the electron affinity of the former beinglower than the energy of the triplet exciton. The high atomic numberelement is closely coupled to the conduction or valence electrons of oneof the semiconductive components, and may be chosen from the transitionmetals able to form square-planar coordinated complexes in which atleast one of the ligands is an acetylenic group. The photoresponsivematerial can comprise the combination of fullerene with a transitionmetal square-planar coordinated complex. There has also been described adevice in which the photoresponsive material is arranged between twoelectrodes which can be used to apply an external bias voltage and whichcan be used to collect current in an external circuit. One of theelectrodes is formed by a transparent conductive layer, e.g. of indiumoxide or indium/tin oxide, or a conductive polymer such as polyaniline,polypyrrole or polythiophene, or derivatives of the same (alloxidatively doped to a conductive state). The other conductive electrodecan be formed by a metallic layer including aluminium, magnesium,calcium, lithium and related metals, any of which may be alloyed withother metals including silver. The invention also contemplates a methodof manufacture of the photoresponsive material in which the severalcomponents are co-soluble in a common solvent, such as toluene,chloroform or related organic or organo-halogen solvents. Thesemiconductive layer can be processed from solution directly onto one ofthe conductive layers which is itself formed on a substrate layer, e.g.a transparent glass. The substrate could be a flexible material such asthin film of a polymer such as poly(ethylene terephthalate).

What is claimed is:
 1. A photoresponsive material comprising:anorganometallic complex comprising a transition metal; and a fullerene, aconjugated polymer with an electron withdrawing group in the main chain,a conjugated polymer with an electron withdrawing group attached as aside chain, a polymer comprising an electron withdrawing oxazole group,a polymer comprising an electron withdrawing oxadiazole group, amolecule comprising an electron withdrawing oxazole group, or a moleculecomprising an electron withdrawing oxadiazole group.
 2. Aphotoresponsive material according to claim 1, wherein theorganometallic complex comprising a transition metal is a semiconductor.3. A photoresponsive material according to claim 2, wherein thesemiconductor is Pt-poly-yne.
 4. A photoresponsive material according toclaim 2, wherein the fullerene is C₆₀ /C₇₀.
 5. A photoresponsivematerial according to claim 2, wherein the conjugated polymer with anelectron withdrawing group in the main chain is poly(pyridine),poly(pyrazine), poly(pyridine vinylene), or poly(pyrazine vinylene). 6.A photoresponsive material according to claim 2, wherein the conjugatedpolymer with an electron withdrawing group attached as a side chain ispoly (p-phenylene vinylene) with nitrile groups attached to the mainchain.
 7. A photoresponsive material according to claim 1, wherein thefullerene is fullerene C₆₀.
 8. A photoresponsive material according toclaim 1, wherein the organometallic complex comprising a transitionmetal comprises a square planar coordinated complex with four ligands,at least one of which is an acetylenic ligand.
 9. A photoresponsivematerial according to claim 8, wherein the transition metal is platinum.10. A photoresponsive device comprising:a first electrode, a secondelectrode; and arranged between the first electrode and the secondelectrode a photoresponsive material comprising: an organometalliccomplex comprising a transition metal; and a fullerene, a conjugatedpolymer with an electron withdrawing group in the main chain, aconjugated polymer with an electron withdrawing group attached as a sidechain, a polymer comprising an electron withdrawing oxazole group, apolymer comprising an electron withdrawing oxadiazole group, a moleculecomprising an electron withdrawing oxazole group, or a moleculecomprising an electron withdrawing oxadizole group.
 11. A deviceaccording to claim 10, wherein the first electrode comprises atransparent conductive layer.
 12. A device according to claim 11,wherein the transparent conductive layer is comprises indium oxide;indium/tin oxide; or a conductive polymer comprising a polyaniline, apolypyrrole, or a polythiophene.
 13. A device according to claim 10,wherein the second electrode comprises a metallic layer.
 14. A deviceaccording to claim 13, wherein the metallic layer comprises aluminum,magnesium, calcium, lithium, or an alloy of one of these metals withsilver.